Preheat oven to 400ºF. Line 2 large rimmed baking sheets with parchment paper. Combine Brussels sprouts, 1 tablespoon sesame oil and ⅛ teaspoon salt in a large bowl; toss until evenly coated. Divide between the prepared pans, spreading in a single layer. Roast until tender and deeply browned, 20 to 25 minutes, stirring and rotating the pans from top to bottom halfway through.
Meanwhile, combine ½ cup kimchi, 3 tablespoons water, 2 tablespoons olive oil, 1 tablespoon vinegar, 1 teaspoon fish sauce, ½ teaspoon ginger and the remaining 1 tablespoon sesame oil in a blender; process until smooth, about 30 seconds.
Transfer the Brussels sprouts and ½ cup of the vinaigrette to the large bowl; toss until evenly coated. Drizzle with the remaining vinaigrette; top with ¼ cup scallions and 2 teaspoons sesame seeds.
